To define the mechanism of integrative recombination the structure of the products of this reaction has been determined. These products are supertwisted circles of DNA that are linked to each other in a catenate. In addition, the pathway of relaxation of supertwisted DNA by a recombination protein has been resolved. This topoisomerase works by transient cleavage of single strands of DNA. A model for integrative recombination is proposed that accounts for these observations and for other features of the biochemistry of this reaction.